Costal Navigation

Coastal Navigation

In land navigation, we use the National Grid to define position. In sea navigation Latitude and Longitude are used

The lines of Latitude, or “parallels”, run around the Earth, parallel to the Equator. They are designated by their angular distance (as measured from the Earth’s centre) up to 90° North or South of the Equator. The latitude scale is on the east and west sides of a chart. Each degree is divided into sixty minutes

The lines of Longitude, or “meridians”, run North/South, from pole to pole. They are designated by their angular distance up to 180° East or West of the meridian which runs through Greenwich, near London. The longitude scale is on the top and bottom margins of a chart. Latitude is also used to measure distance. The distance on the surface of the earth represented by one minute of Latitude is known as a Nautical Mile or Sea Mile, which is divided into 10 Cables (one cable = approximately 200 metres). Six Sea Miles is about 7 Land Miles. The latitude scale on either side of the chart is used as a distance scale. In the diagram below, the divider is measuring a distance of 3 nautical miles. Speed is expressed in nautical miles per hour. One nautical mile per hour is called a “knot”.

Chart Symbols Like the Ordnance Survey maps that you have already learned to use, charts use symbols to represent different features. Charts show lighthouses, navigation buoys and their lights, shoals, sandbanks and rocks, as well as features on the land which can be useful for coastal navigation. A selection of various symbols commonly used in Admiralty charts is given on the following page.

NEVER use the longitude scale (top and bottom of the chart) to measure distance!

Coastal Navigation

Depths and heights On a land map the height of the land is important, but on a chart the depth of the water is important. All over the sea areas of the chart you will see figures indicating the depth of water in metres. In shallow water there is often a small figure after the ordinary size figure – e.g. 33. This means a decimal – 3.3 metres.

Because of the regular rise and fall of the tides, the depth of water is continually changing. The lowest low tide that can be calculated is the level from which the soundings are measured, so the water depth marked on a chart is the least depth that can usually occur at that place. In the green areas of the chart, covered by high tide, but exposed at low tide, the figures are underlined, indicating height above the level of lowest Low Tide. They are called Drying Heights. For example, 17 means that this spot will not be covered by water until the tide has reached 1.7 metres.

Heights of land, hills, islands, rocks which never cover, lighthouses and beacons, as well as clearances under bridges or overhead cables, are measured from the height of High Water Springs.

Coastal Navigation

Plotting Position Take a bearing on something you cn see clearly and can identify on the chart (eg. lighthouse). Line the parallel ruler through the bearing mark on the outside ring and the centre of the compass Rose. “Walk” the ruler across the chart making sure that it does not slip, until the edge touches the symbol of the lighthouse on the chart. Draw a line on the chart along the edge of the ruler from the lighthouse. Your position is somewhere on that line.

The Breton Plotter has a rotating dial in the middle which acts as a mobile compass rose. Rotate the dial until the correct bearing is at the zero mark at the end of the instrument marked with an arrow. Make sure that the North mark on the dial is pointing to the north of the chart and that the grid lines in the centre, line up with either latitude or longitude lines on the chart. Keeping these in line, move the instrument until the edge touches the Lighthouse. Then draw in the bearing line.